I spoke to some of my friends in the past and asked them why they quit the game. A lot of them said that first of all the leveling felt like a total grind they couldn't get their minds past, so some of them didn't even reach inferno.
So there's a couple of things you can do to make both inferno farming and leveling (1-60) more interesting.
1) Increased mob density, also in mp 0 and in normal - hell. First I thought Ok mob density is not needed in normal - hell and mp0 because lower mob density is part of the learning curve and progression. So MP1-10 could be enough to increase mob density. But in the end, everyone found normal and nightmare way too easy so think it could make the leveling part much more interesting as well.
2) Increase base movement speed. Not only are those first acts like ghost towns, while leveling through normal-hell you also have less movement speed, which makes it take even longer to find a new pack of mobs. As a result of increasing base movement speed, you could probably nerf skills like sprint and tactical advantage by 10% or something, so it feels less mandatory for any farming build and brings barb/dh slightly more on par with the rest of the classes in terms of farming speed.
3) Instead of trying to equalise everything, why not bring different purposes to everything? or even better, first try equalise everything within a certain margin, and then add things that can only be obtained in certain parts of the world. Or things that can only be done on certain classes.
The problem is if everyone gets the choice which act they find most fun and most rewarding they will keep doing that until they are sick of it and then just quit the game because there's no motivation to go in another act they find "less fun or less rewarding". But if they are forced /motivated to go there anyway, they will realise it's actually much more fun than act 3, which they did 500 times.
Since I no longer care for exp after I reached lvl 80, I started farming act 1 and 2 again even though I feel like I'm missing out on loot and they are not my favorite acts. But boy those acts are sooo much more fun because I'm sick of act 3. If only there was more motivation to go there, for example a higher tear of gems dropping only in act 1 and a chance for a second demonic essence from elites only in act 2, and account bound fiery brimstones dropping from bosses in act 4 for example. Then people don't care if there's 20% less exp and loot to be gained if they are desperately in need for gems or brimstones for their crafts.
This way Blizzard, for you it's much easier to keep things "balanced". By trying to keep every act and every class equalized you are making it hard for yourself and if there's one thing I've seen, it's that the team failed over and over again to succeed in their goal to equalize everything. And it's not that the D3 team is a bad team, equalizing everything is just a very very hard thing to balance...
The same with classes btw. Most people are playing only one character, because other characters have the same purposes as your main. Farming for loot. And since it's your main that has MF through paragon levels, there's no point to make alts. But if you could introduce some system where different characters have different purposes. It would take at least 2-3 characters to be able to fulfill every role and people are motivated to make more characters.
People don't do things they think won't be fun. But if they are forced/motivated to do it anyway, they'll usually be having much more fun then they thought they would have. That's where the fail is in diablo 3. You made a game that sounds good in theory by making it easy and taking away depth. By taking away permanent decisions like stat/skill allocation and by making it so that one character is good for everything. Well, that's how you make a BORING and IMBALANCED game.